The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Metal comb structures that include an “A” comb inter-digitated with a “B” comb are known. For example, such structures have been widely used in wafer fabrication to capture minute conductive defects that may kill the device or create a short between two separate interconnects of the wafer.
Generally, metal comb structures including an A comb and a B comb are such that each of the A comb and B comb include a number of metal fingers that are inter-digitated. That is, the metal fingers of one plate are interposed between the metal fingers of the other plate. These fingers, sometimes referred to as even metal lines and odd metal lines, are relatively thin and generally quite long. Furthermore, the spacing between the fingers is quite narrow, somewhat dependent on the design requirements in a particular instance.
The high aspect ratio of these structures, dictated by the height of the fingers and the narrow spacing between the fingers, generally results in some disadvantages when it comes to failure analysis de-processing, and particularly when it comes to locating physical defects in the plate comb structures. Insufficient isolation methods inevitably lower failure analysis success rates and lead to increased cycle times.
One method that has been used extensively for failure analysis is the OBIRCH (optical beam induced resistance change) method. This method has been an indispensable failure analysis tool in the semiconductor industry for many years. The method has found use not only for test structures, but also for final products, and is useful for field failures and failures in manufacturing processes at the development phase and mass production phase.
The principle behind the OBIRCH method is relatively simple, generally including heating a structure and detecting resistance change. However, the method has many features and a wide field of application as would be readily appreciated by those of skill in the art.
The sensitivity of defect detection using the OBIRCH method has been improved over the years in various ways. For example, it has been shown that OBIRCH signal is stronger at lower temperatures. The use of a constant current source, rather than a constant voltage source is known to make OBIRCH more sensitive in some applications. It has also been shown that a combination of laser-modulation and lock-in amplifier has made the method more sensitive. Sensitivity may also be improved by using a pre-amplifier.
Derivatives of the OBIRCH method have also been shown to be useful in failure analysis. For example, the Seebeck effect has be shown to work well for detecting voids when no bias is applied to a device. Schottky effect has also been shown to work well for detecting abnormal semiconductor contacts.
Generally, for the detection of failure in a metal comb structure, passive voltage contrast in the scanning electron microscope (SEM) is needed to find the defect more quickly and more accurately than using the OBIRCH method alone.
As the space between metal fingers or lines within metal comb structures decreases, difficulties arise when attempting to detect defects, such as metal stringers, between the fingers or lines. This is generally due to the limited depth of focus of the failure analysis in-line optical analysis tools currently available. Generally, there is a heavy reliance on electrical fault isolation and the accuracy of finding the minute defects using the SEM.
As such, there has been a growing need to develop systems for identification of defects and locating these defects in metal comb structures.
In some instances, a reactive ion etch (RIE) tool has been employed to de-process structures and expose metal lines. Subsequently, the short or defect location has been determined. In that case, oxide particles and TiN shorts may be easily etched away by the RIE tool which generally uses an F-based gas for oxide etching. Indeed, in some instances failure analysis findings have been incorrect due to a misinterpretation of an artifact that was produced during the de-processing process.
The present disclosure aims to provide an alternative metal comb structure that is designed to facilitate easy identification of defects during failure analysis operations.